Indicator for frequency modulated radio locator systems



Nov. 26, 1946. v R. c. SANDERS, JR 2,411,595

, INDICATOR FOR FREQUENCY MODULATI ID RADIO LOCATOR SYSTEMS Original Filed July 26, 1943 3 Sheets-Sheet 1 frjwm W f r INVENTOR.

Nov. 26, 1946. 2,411,595

INDICATOR FOR FREQUENCY MODULATED RADIO LOCATOR SYSTEMS R., c. SANDERS, ,JR

Original Filed July 26, 1943 3 Sheets-Sheet 2 THE nmlvirxcfmi mwrm 040560 0709mm: la/rams} I N VE'V TOR.

m dmiwm BY It Nov. 194& R; c. SANDERS; JR

INDICATOR FOR FREQUENCY MODULATED RADIO LOCATOR SYSTEMS Original Filed July 26, 1943 3 Sheets-Sheet 3 mrrrzarwfa/ama Patent UNITED S AT S PATE are INDICA'IbR FOR FREQUENCY MODULATED RADIO LOCATOR SYSTEMS Royden C. Sanders,

signor to Radio Corporation partition of Delaware 1 This application is a division of my copending f1 Claims. (Cl. 172-245 Jr., Chestnut Hill, Mass, as

of America, a cornetic bias as described hereinafter, if there are United States application Serial No. 496,246, filed July 26, 1943, entitled Frequency modulated radio 4 locator systems and indicators.

My invention relates to object locating and distance measuring systems of the type utilizing the transmission and reflection of radio waves or of other suitable waves. The invention relates particularly to multiple frequency indicators and to their use in an object locating system of the frequency-modulated type whereby simultaneous indications of a; plurality of reflecting objects may be obtained.

v Distance measuring systems of the frequencymodulated type are described in Bentley Patent 2,011,392 and in Espenschie'd Patent 2,045,071. In these systems, the radiated frequency-modulated wave is reflected from the earth's surface or from an aircraft or other object to be located and the reflected wave is received by means in-' cluding a heterodyne receiver located in the vicinity of the transmitter. The heterodyning or mixing signal for the receiver is obtained directly from the transmitter wherebythe receiver output includes a signal of beat frequency which frequency is a function of the time required for the radiated signal to reach the reflecting object and return to the receiver.

99 discs (and 99 reeds), the first disc may have two sine waves cut on it, the second disc three 4 sine waves, etc., the last'disc having 100 sine waves cut on it. Thus, if rotated at the rate of 1 50 rotations per second, the group of discs will produce magnetic force pulsations at the reeds number 1 to 99 having pulsation rates or frequencies from 100 to 5000 per second, respectively.

- Assuming that the indicator is employed in a frequency-modulated distance measuring system, the visual frequency indications are obtained by applying the beat-frequency output signal of the detector (after suitable amplification) to the indicator in such a way as to vary the field between the several discs and their associated reeds. This detector output signal'may include frequencies from 0-5000 per second, for example. Thesesi nal pulsations beat with the pulsations caused by the sine wave configuration of the discs whereby the field pulsations at one particular reed will fall within the response range of the reeds, as-

' sumed to be from 0 to 60 cycles, and this reed will vibrate or deflect. Since the field pulsations at this one reed have been produced by a particular beat-frequency signal in the detector output,

An object of the inventionis to providean improved multiple object or target indicator for I object locator systems of the cyclic frequencyare tuned to respond to driving forces of the same frequency or narrow band of frequencies. For example, each reed may be deflected or vibrated by any appliedforce, such as a magnetic field or an electric field, that varies in intensity at a rate of from 0 to 60 cycles per second. Adjacent to each reed there is located the P riphery of a rotatable disc which has a sine wave curveon'the periphery for varying the magnetic or electrostatic pull between the disc and the reed. A large number of reeds and discs may be provided, the number depending upon the frequency range to be covered and the accuracy desired. Each disc has a different number of sine waves cut on the periphery. For instance, assuming the use of magthe reed vibration indicates the presence of this particularbeat-frequency signal and, therefore, the presence oi-a reflecting object a. corresponding distance away.

The invention will be better understood from the following description taken in connection with the accompanying drawings in which Figure 1 is a diagram of a frequency-modulated radio locator system which includes a multiple target indicator constructed in accordance with one embodiment of the invention,

Figure 2 is a view taken on the line 2-4 of Fig. 1, Y

Figure 2a is a plan view of one of the tuned reeds used in the indicator of Fig. 1,

' Figure 3 is a view taken on the line 3-3 of Figure 4 is a group of graphs that are referred to in explaining the invention,

Figure 5 is a side viewof an electrostatic type indicator embodying the invention, together with 6 is a view taken on the line 6+6 of Fig.

In the several figures, like parts are indicated by the same reference character.

Fig. 1 shows an embodiment of the invention as applied to a frequency-modulated distance measuring system comprising a. radio transmitter Ill and a frequency-modulator H for cyclically mod-- ulating the transmitter carrier wave in'accordance with a modulating signal such as the triangular wave. I 5 (Fig. 4) that is supplied to the modulator from a pickup device l2. The frequency-modulated wave is radiated from an antenna it to the reflecting objects and the reflected signalis picked up by a receiver antenna it and supplied to a beat-frequency detector it. A heterodyning signal is also supplied directly from the transmitter Hi to the detector [6 where it bwts with the reflected signal to produce a beatfrequeny signal in the output circuit of the detecis designed in accordance with one embodiment of the invention.

Referring to Figs. 1 and 2, the indicator l8 comprises a' plurality of discs A, B, C, D, E, etc., which are made of steel or other magnetic material. These discs are mounted on a shaft 2|, also of magnetic material, which is rotated by a motor 22. The speed of rotation may be 50 rotations per second, for example. Each of the discs A, B, C, etc., has a sine wave configuration cut on its periphery. This is illustrated in Fig. 2 where the disc D is shown.

A plurality of tuned reeds A, B', C, etc., are

-mounted adjacent to the peripheries of the discs A, B, C, etc., respectively, each reed being tuned for maximum response at thesame frequency or narrow band of frequencies. For instance they may be tuned for maximum response at and in theregi'on of 25 cycles per second, the reeds being' damped as indicated in Fig. 211 so that the reed response resembles that of a low pass filter.

on a supporting bar 23 which may be bent backon itself to provide a section 23' upon which the input coil is is wound. Both the indicator reeds between the motor speed and the frequencyand the bar 23 are of magnetic material so that cuit for the coil It.

each reed varies sinusoidally as tions introduced by one particular disc. The rate of this variation at each reed depends upon the number of sine waves cut on. the associated disc. Since these pulsations or variations in the magnetic field occur at frequencies that are higher than the frequency response range of the tuned.

reeds, the reeds are not deflected thereby. Such pulsations will have a range of from 100 to 5000 cycles per second, in th example assumed.

Upon the reception. of waves from reflecting objects located at different distances from the transmitter, the resulting beat-frequency signals will flow through the coil 18 and produce at the tuned reeds fluctuations in themagnetic field. These fluctuations will beat with the fluctuations producedbythe rotating discs and produce beat fluctuations in the magnetic field.

These beat fluctuations are magnetic force beats that deflect or vibrate the reeds. The graph W of Fig. 4 illustrates such a beat fluctuation produced by the signal 20 and by the disc that caused the field variation represented by the graph 26. Since the beatefrequency'signals from the detector !6 lie within the range of from 0 to 5000 cycles per second in the example assumed, there will be beat fluctuations in the magnetic force of the required low frequency (0 to cycles) at certain indicator reeds to cause their vibration or indication due to variations in the speed of the motor 22, it is desirable to maintain synchronism modulating signal. This may be done conveniently by permanently recording the modulating signal, such as a triangular wave, on a magnetic tape 26. The tape 26 may be carried by a nonmagnetic wheel 21 (Fig. 3) that is keyed or other-' wise fastened to the shaft 2|. The modulating signal is taken off the tape 26 by the pickup unit I 2 and supplied to the modulator I I.

With this arrangement, any variation in motor speed will change the frequency modulation rate 1m and cause a change in the frequency f: of the received signal reflected from a target a given distance away. This is shown by the equation (t.) (D) (B) p where D is the distance to the target in feet and The indicator may be operated either with or the coil 08 will be the D. 0. component of the aud'ro amplifier plate current. 'Inthat case, if the discs A, B, C, etc., were stationary, there would. be a steady magnetic field between the discs and the associated tuned reeds and .a steady magnetic pull on the reeds. When the discs are rotated, however, the magnetic force exerted on Bis the band width of the frequency modulation in megacycles, thefrequencies fr and in being in cycles per second. Therefore, while any motor speed increase, for example,-will increase the rate of the'magnetic field pulsations caused by the discs A, B, etc., it will cause a like increasein the frequency of the field pulsations produced by the signal from a given target and the beat frequency of these two pulsations will remain unchanged.

In practicingsthe invention, the indicator may be operated either with a bias of magnetic flux as assumed in the foregoing description, or with no bias, 1. e., with no D.-C. component in the flux. In either case the beat-frequency discontinuity at the end of a frequency modulation sweep, at times, may be such that the beats produced during one sweepoppose the tuned reed illustrated in I Fig. 4 where the graph 25 represents thevvariaequal to twice the signal .sweep. This might result in poor indication for a particular target if such discontinuity remained the same for each successive sweep. However, this would be the case only if the reflection were from a strictly stationary target.

The more usual situation will be where the indicator is used to locate moving objects such as surfaceships or aircraft. In that case, the beatfrequency discontinuity will be different atthe end of each successive frequency-modulation sweep and nodlscontinuity of the type causing transformer 31 whereby the beat-frequency signal voltage varies the electric field between the whereby the tuned reed opposite a particular disc will be vibrated by a particular beatfrequency signal just as described in connection I with Fig. 1.

reeds respond to frequencies from to 60 cycles per second and where the frequency modulation is at the rate of 50 per second. If the tuned reeds have maximum response in the region of 25 cycles per second, then the received signal that will give maximum reed response will produce only about a half cycle of magnetic force to actuate the reed, i. e., there will be only this fractional cycle before the next discontinuity'occurs in the beat frequency. This is not objectionable because the reedscan be swimg to full amplitude of deflection in a half cycle or less.

There are certain differences in the design and operation of an indicator having magnetic biasand one having no magnetic bias which are noted below.

With magnetic bias 7 In this case the frequency of themagnetic force (thematnetic force beat) that vibrates a reed is equal to the signal frequency minus. the sine wave disc frequency.

As to the efiect of a beat frequency discontinuity, any discontinuity tends to impair the tuned reed response somewhat.

Without magnetic bias In this case the frequency of the magnetic force (magnetic force beat) that vibrates a reed is frequency minus the sine wave disc frequency. This means that the numher of sine waves cut on each disc should be twice the number of sine waves cut thereon for use with magnetic bias; Thus a varying force such as shown by the graph 40 in Fig. 4 beats with the varying force caused by thesignal 20, for example, to produce the graph 45.

beat-frequency discontinuity The effect of a differs from the case where bias is used in that a 180 phase change has no effect. Any other discontinuity, however, impairs the reed response somewhat. 1

Figs. 5 and 6 show an embodim'ent ofthe invention that differs from that of Fig. 1 in that a varying electric force instead of a magnetic force is utilized for driving the indicator reeds A, B, etc. In this design the discs A, B, etc.-, and the other parts which should be of magnetic material in the design of Fig. 1 need only be of conducting material so that a. suitable voltage difference may exist between a disc and its associated reed. A D.-C. voltage such as 500 volts may be applied to the indicator reeds through I thesecondary of a transformer 31 while the ground mtential oil through '2.

the beat force representedbybeing sufficient to reduce the. electric field at the reeds to zero. If the indicator of Fig. 5 is to be operated with no bias, the lower end of coil 36 is grounded and the number of sine waves on the discs are doubled.

The response of the indicator reeds to diiferent beat-frequency signals is illustrated in Fig. 'I for the particular design constants that have been assumed and for the case where there is either a magnetic bias or an electric field bias. The graph :38 shows thatthe reed opposite the 450 cycledisc (9 sine-waves on disc) will respond to applied beat-frequency signals over the comparatively narrowirequency range from 390 cycles per second to- 510 cycles per second. If the beat frequency of the applied signal is 450'cycles,' there will be a zero force beat and thereed will b deflectedaway from its center position. If the beat frequency of the applied signal is either 425 cycles or 4'15 cycles, for example, the force beat will be a 25 cycle beat andthe reed will vibrate with the same response in both cases.

It will be noted from a comparison of the graphs 48, 49 and "5,! that the response ranges of adjacent reeds are made to overlap. Therefore, while the reed opposite the 450 cycle disc responds alike to applied heat-frequency signals of frequencies above or below 450 cycles, it is tuned comparatively sharply to-have a response such as shown by the graphs 4813,1911 and Slb.

Itwill be noted that the response ranges for ad- :Iacent reeds overlap so that there is nodead spot" in the frequency response of the indicator.

' I claim as my invention: 7 I 1. A frequency meter comprising a plurality of indicating elements each of which is tuned to respond to the same; narrow band of frequencies, means for applying to all of said indicating ele ments an electrical force that varies in accordmechanical ance with an applied signal containing the frequency components to be indicated, and'means for varying said electrical force periodically at each of said indicating elements and at a. different rate at each element for'producing force beats which occur at a repetition rate that, at at leastone of the indicating elements, is within the said narrow response range or the indlcat-' ing elements.

2. A frequency. meter comprisingaplurality of indicating elements each of which is tuned to respond to the same narrow band of t s rrequencies; means for applying indicating elements an electrical force that varies o erates to 'all of we in accordance with an' applied signal containing the frequency components to be indicated, and

means for varying said electrical force sinusoidally at eachof said indicating elements and at c.

difierent rate at each element for producing force beats which occur at a repetition rate that, at at least one of the indicating elements, is within the said narrow response range or the indicating elements. t

3. A frequency meter comprising a plurality of v of indicator reeds each tuned to have the same frequency response, means for applying a perlindicating elements each of'which' is tuned to re- A spond togthe same narrow band of frequencies,

means ior'applying to all of saidindicatlng ele- 'ments an electrical biasing force and an'electrical force that varies in accordance with "an applied signal containing the frequency compo-- nents to be indicated, and means .ior varying said electrical force periodically at each of said indicating elements and at a different rate'at each element for producing 'force beats which occur at a repetition rate that, at atleast one ofthe indicating elements, is within the said narrow response range of the indicating elements.

4. A frequency meter com rlsing a, plurality of indicating elements each oiwhich is tunedto respond to the same narrow handof frequencies, means {or applying to all of sad indicating elements an alternating current electrical force only that varies. in accordance with an applied signal frequency response. means for 'awly ns & P

crllc force to each of said reeds with the periodic rate different at each 01 said'reeds. and means for varying said periodic force in accordance with an applied signal whereby beats are produced in the forceapplied to saidreeds with the frequency or said beats at one oi the reeds lying W thin'the frequency band to which said reeds are responsive.

v6, A frequency indicator comprising 'a plurality odic force to each or said reeds with the periodic rate different at each of said reeds, and means containing the frequency components to he indicated, and means for lva rying said electrical force periodically at eachoi said indicating elements for varying said periodic force in accordance with an applied signal containing a plurality of fre quency components whereby a plurality of beats 1 are produced in the forces applied to said reeds, one of said beats having a frequency lying within the-frequency band to which said reeds are responsive andoccurring at one of said reeds where'- by it is deflected.

7. .A frequency indicator comprising a plurality oi rotatable discs each having its Pflphery'cut in the configuratlonof a. sine wave and each disc having 'a difierent number 01' sine waves cut thereon, .a plurality or similarly tuned indicator reeds positioned adjacent to the peripheries of said discs, respectively, means for establishing a force between said discs and said reeds which varies with the change in spacing therebetweenas the discs are rotated, and means for further varying said force in accordance with the, ape plied signal to be indicated for producing: beats in said iorcevwh'erelby a given frequencycompo- I nent'in the applied signal produces beats amering in frequency at the diflerent reeds, one. of

' said heats having a frequency within the response range of said reeds whereby it'deflects the reed that is located where said one heat occurs.

' ROYDEN C. fiANDERS, JR. 

